There has been increased interest in the clinical application of Class Ill (K+ channel blockers) as anti-arrhythmic agents. Unfortunately, reduction of the HERG current or other slow delayed rectifiers has been demonstrated to be proarrhythmic, both in abnormalities in coding genes or following pharmacologic intervention. This has placed renewed interest in the rapidly activating K+ currents that are important during the earlier phases of the cardiac action potential in various regions of the heart. However, K+ channel blockers vary in their selectivity for each K+ channel type, and display a variety of conformation specific interactions with K+ channels. Such conformation specific interactions can cause the degree of block of a channel to vary by orders of magnitude depending on the pattern of electrical stimulation. Such conformation dependent binding can be either detrimental or beneficial. This proposal focuses on examining the relationship between antiarrhythmic drug binding and a particular class of conformation changes, namely C-type inactivation in cardiac K+ channels. C-type inactivation is more widely distributed among cardiac K+channels than N-type and may be the dominant determinant of such important properties as recovery, K+sensitivity, pHo sensitivity and drug use-dependence. The goal of this proposal is to elucidate how C-type inactivation can influence the complex patterns of block and use-dependence seen with Class III agents in the cardiac channels, Kv1.4, Kvl.5 and Kv4.3. The main hypothesis of this proposal is that C-type inactivation involves a rearrangement of the intracellular pore resulting in its closure or partial closure. We further hypothesize that rotation of S6 is a critical event in this process and accounts for coupling of intracellular closure to the extracellular conformation changes in the S5-H5 linker region and the H5-S6linker. Our goal is to combine the structural information emerging from the new crystal structure with kinetic and biophysical measurements with computer modeling to achieve a detailed understanding of blocker-channel interactions. This study will investigate the intracellular and extracellular changes that occur during C-type inactivation and examine how these changes alter drug binding, accessibility and recovery. This study will provide a molecular basis for the use dependent properties of a broad class of cardiac ion channels and drugs that will further the development of safer and more effective anti-arrhythmic drug therapy.